It's important to get the right base!
4 Light Bulb Base Types To Know About
Updated on Feb. 26, 2024
How Do I Know What Base My Light Bulb Has?
It’s all in the name.
In lighting, the numbers in the name tell you what base you need for your replacement bulbs, called “lamps” in the electrical trade. Every lamp base has a standardized code, like E12 or G4, that describes the base style (the letters) and a measurement in millimeters. The latter is often the diameter of the base, but not always.
“This standardized naming convention helps users match bulbs with the correct socket size in fixtures, ensuring compatibility between bulbs and holders,” says Joel Worthington, president of Mr. Electric residential and commercial electricians.
If you’re standing there with a broken light, check the label. If it’s not visible, we’ll tell you what to look for. (You’ll need a ruler.)
I’ve swapped out so many lamps in my years as an electrician that I feel like I know them by heart. But I haven’t done much residential work, so Worthington’s here to walk us through the most common household light bulb bases, and how to know which ones you have.
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Edison Screw
When you think of light bulbs, this base probably comes to mind.
Named for Thomas Edison and patented in 1881, the metal, grooved base screws into the light socket to make the electrical connection that powers the light. The metal threads connect to the neutral, while the bottom contact point makes the “hot” connection.
Edisons have an “E” prefix and come in multiple sizes, most commonly the E26, which means the base is 26 millimeters in diameter. This size, interchangeable with the European E27, is the one you see in incandescent, light-emitting diode (LED) and compact fluorescent lamps (CFL) everywhere.
Other sizes are E17, called “intermediate,” and the E12, aka candelabra. There’s also the E39, called the “mogul” base, which is used for things like streetlights and other high-wattage applications.
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Candelabra
Candelabra bases are actually Edison bases, but they’re often listed as a separate category. If you’re looking to replace the lamps in a chandelier or fancy wall sconce, you’re probably looking for bulbs with candelabra bases.
Candelabra bases are 12 millimeters in diameter— smaller than other Edison screw bases— and are thus called E12s.
Candelabra base light bulbs come in several globe styles, like round, pointed, flame-shaped and even tubular. And while you’re replacing the lamps, consider changing to LED. Changing a chandelier bulb isn’t something you want to do often, and LED bulbs last longer and are more efficient.
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Bi-Pin
Lamps with bi-pin bases get pushed, not screwed, into position.
Two metal prongs, pins or loops project from the base and extend into the bulb to make the electrical connection and light the lamp. This category is a large one, but Worthington says homeowners often see these bases on the small halogen bulbs used in accent lighting.
Look for names like G4 and G9, which indicate the prongs are 4- and 9-mm apart, respectively. (Worthington says the “G” stands for glass.) When shopping for replacements for these small bi-pins, bring one with you to the store if you aren’t sure which size to get. Usually, there’s a true-to-size diagram on the package that you can hold the bulb up to.
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Twist-Lock
This subset of the bi-pin base category has “GU” prefixes. They have two prongs like all bi-pins, but there’s a twist and lock function that secures the lamps in place.
Worthington says these bases are often found on specialized lighting fixtures and technologies, from track lighting and spotlights to CFLs and LEDs. Common examples of these bases are GU10 and GU24.
“GU10 is a type of bi-pin base often used in track lighting and recessed fixtures,” Worthington says, “while GU24 is a twist-and-lock base designed for energy-efficient bulbs in certain residential and commercial applications.”
About the Expert
- Joel Worthington is the president of Mr. Electric, a Neighborly company.
